A mar is a flaw, mark, or blemish, like a deep scratch on a wooden table. As a verb, to mar is to make such an imperfection — like the pen mark that mars your crisp, white shirt.
We often think of things that mar as immediately noticeable, like scratches, scars, and blemishes, but sometimes mar describes other ways of ruining something. For example, an unexpected run-in with a difficult person can mar your relaxing day at the beach, just as an unpleasant memory can mar a peaceful state of mind. The thing to remember is, it only takes one scratch, one mean comment, or one negative experience to mar something that is otherwise fine.